The present invention generally relates to a nonvolatile semiconductor memory device, and more specifically, to a semiconductor memory device comprising a three-dimensional cell array to reduce chip size.
A nonvolatile ferroelectric memory, for example, a Ferroelectric Random Access Memory (FeRAM) device has attracted considerable attention as a candidate for the next generation memory device, because it has a data processing speed as fast as a Dynamic Random Access Memory (DRAM), and it conserves data even after the power is turned off.
An FeRAM having a structure similar to that of a DRAM includes capacitors made of a ferroelectric material, which has a high residual polarization, allowing retention of data after power is turned off.
A unit cell of a conventional nonvolatile FeRAM device includes a switching element and a nonvolatile ferroelectric capacitor. The switching element performs a switching operation depending on a state of a word line to connect the nonvolatile ferroelectric capacitor to a sub bit line. The nonvolatile ferroelectric capacitor is connected between a plate line and one terminal of the switching element. Here, the switching element of the conventional FeRAM is a NMOS transistor, whose switching operation is controlled by a gate control signal.
In the conventional FeRAM, as the cell size becomes smaller, data retention characteristics are degraded. Thus, it is difficult to perform a normal operation of cells. For example, when a voltage is applied to an adjacent cell in a read mode of the cell, data is destroyed due to an interface noise generated between the cells. Also, when a write voltage is applied to an unselected cell in a write mode of the cell, data of the unselected cells is destroyed, thus not facilitating a random access operation.
For Metal Ferroelectric Insulator Silicon (MFIS) and Metal Ferroelectric Metal Insulator Silicon (MFMIS), the data retention characteristics is degraded by depolarization charges. The degradation of data retention characteristics caused by smaller cell size is also a problem for other well-known nonvolatile memory devices such as a phase-change RAM (PRAM) device, a magnetoresistive RAM (MRAM) device, or a resistive RAM (ReRAM) device.